The AD8 would have been on a static IP address. Otherwise the controller wouldn't know where to find it and it wouldn't work (just like what is happening now).
This could have been set in the AD8 or a DHCP reservation in the router. Sounds like it was set in the router that got erased when equipment was replaced.

If there was no documentation that was left for you from the original installer (BS hack) and you have no idea what the original IP address on the network was; then yes you are going to need the Integration Designer software and make the changes to the XP6 to get it to communicate again. And set a static IP in the AD8!